Over the years, various injection molding methods and apparatuses have been provided as methods and apparatuses for obtaining molded products with comparatively high precision from synthetic resins such as plastic.
In particular, among the above-mentioned injection molding methods and apparatuses, in recent years injection molding method and device have been proposed for reducing the molding cycle time for molded products.
This conventional injection molding method or apparatus comprises a nozzle for temporarily holding pre-pressurized molten resin and a valve pin positioned inside the nozzle such that it can be freely inserted or withdrawn. A gate is opened by means of this valve pin to fill the cavity inside the mold with the pre-pressurized molten resin and, after completion of filling the cavity with molten resin, the gate is again closed by means of the valve pin.
To obtain relatively large molded products, conventionally, a plurality of valves having the above-mentioned valve pins are arranged to face the cavity in the mold, and molten resin is injected simultaneously from each gate of the plurality of nozzles into the cavity to fill it.
According to this kind of injection molding method and device, during the time the mold is opened to take out the molded product, the molten resin in the nozzle is pre-compressed. Therefore, the wasted time required for compressing the molten resin until injection is reduced and, as a result, there is the effect that the molding cycle time is shortened.
FIG. 8 is a cross-section diagram outlining the main parts of the above-mentioned injection molding apparatus 1, and particularly showing injection molding apparatus 1 for molding disk-shaped parts having a bearing hole in their centers (for example, molded products such as gears.)
The injection molding apparatus 1 comprises upper mold 2 in which is provided valve main units (not shown in the figure) that pre-compress the molten resin and lower mold 4 in which disk-shaped cavity 3 is formed. Of these upper and lower molds, a plurality of nozzles 5 are provided in specified locations within upper mold 2.
In addition, the configuration is such that a plurality of gates 6, each consisting of a circular hole, is penetratingly formed at the bottom end of each of nozzles 5 and in the upper surface 3a of cavity 3, corresponding to the plurality of nozzles 5, and disk-shaped cavity 3 is filled with molten resin from the plurality of gates 6 simultaneously.
Also, inside of each of these nozzles 5, valve pin 7 is arranged facing gate 6 such that it can be inserted and withdrawn in the vertical direction, and so that the downward movement of the valve pin 7 closes gate 6 and thus stops the injection of molten resin into cavity 3 (so called “gate cut”).
In the center of cavity 3 of lower mold 4, cylinder-shaped core 8 is positioned to form the central hole of the disk-shaped molded product that is formed by the molten resin which fills the mold. Ejector pins 9 are provided around the core 8 so as to be freely movable upward/downward relative to core 8, for removing the cooled/solidified molded product from cavity 3.
Next will be explained the operation of the above conventional injection molding apparatus 1.
Due to the upwardly retracted position of valve pin 7 as shown in FIG. 8, gate 6 is in the open state. Thus, the molten resin which was compressed inside the main body of the valve (not shown in the figure) is injected into cavity 3 via each of the gates 6 of the plurality of nozzles 5 as shown by the arrows.
At this time, as in FIG. 9 which shows a schematic cross sectional view taken at A—A of FIG. 8, molten resin is injected concentrically into cavity 3 from each of the plurality of gates 6 positioned at different locations, as shown by the arrows. These injected molten resins collide and mix so that the cavity 3 becomes filled.
When, the molten resin is injected into cavity 3 via the plurality of gates 6 in this way and filling is completed, valves 7 move downward as shown by the arrows in FIG. 10 and each gate 6 closes. As a result, injection of molten resin into cavity 3 is stopped (so-called “gate cut”).
Afterward, using cooling means not shown in the figure, the molten resin with which cavity 3 has been filled is cooled and solidified.
After the molten resin filling cavity 3 is solidified in this way, the mold is opened by separating the upper mold 2 and lower mold 4, and disk-shaped molded product 10 of solidified molten resin remains in cavity 3 of lower mold 4.
Afterward, when each of the plurality of ejector pins 9 is moved upward parallel to core 8 as shown in FIG. 11, disk-shaped molded product 10 is extracted from cavity 3 of lower mold 4, while its lower face is supported by ejector pins 9.
With this kind of conventional injection molding apparatus 1, as revealed in FIG. 12 which shows an elevation view of molded product 10 and FIG. 13 which shows a plan view of molded product 10, molded product 10 (for example, a flat gear) is integrally formed into a disk shape with bearing hole 10A in its center.
However, with the above-mentioned conventional injection molding method and device 1, as shown in FIG. 9, molten resin is injected in cavity 3 in a concentric manner as shown by the respective arrows from each of the plurality of gates 6 positioned in different locations. The cavity 3 is filled as the molted resin collides and mixes within each injection from respective gates 6, but the molten resin injected from different gates 6 does not mix each other under uniform melt conditions in cavity 3. As a result, there is a danger that molten resin boundary regions will be generated in cavity 3 by the molten resin injected from each of the plurality of gates 6 positioned in different locations.
If such molten resin boundary regions are generated in cavity 3 by the molten resin injected from the plurality of gates 6 positioned in different locations, then, as shown in the elevation view of FIG. 12 and the plan view of FIG. 13, shrinkage action begins at each boundary region in the molded product the is extracted after being cooled and solidified. Due to the shrinkage action, deformed areas 10b and 10c, or so-called “sinks”, occur on the surface, etc. of molded product 10. This can be a major cause of dimension errors in molded product 10.
Since molded products in which sinks occurred must be re-processed for dimensional correction, there is the defect that manufacturing cost of molded products becomes extremely high.
With the foregoing in view, an object of the present invention is to provide an injection molding method and device which reduce the dimension errors of molded products as much as possible.